The retention of aerosol particles (10 M to submicron effective diameters) in the respiratory tracts of laboratory animals will be studied. Measurements will be made of casts of the entire tracts (nose through lung). Deposition by impaction, settling, and diffusion will be calculated utilizing the morphometric data. Toxicological and microscopic studies will be made in an attempt to test the predictions of retention resulting from the calculations. Total retentions and regional retention (noe, upper respiratory, and respiratory regions) will be studied. Retaineddoses by inhalation (LD50 or sublethal effective doses) will be compared to doses by other routes, e.g., injection oringestion. Locus and extent of retention in the laboratory animals will be compared with that expected in man as a function of aerosol particle size. Lung casts will be made with epoxy resins, acrylics and other suitable materials. Heterodisperse and monodisperse aerosols will be generated by techniques now in use in this laboratory. Particle sizes will be measured by cascade impactors, cyclones, optical techniques and electron microscopy. Calculations of impaction, settling and diffusion will be made by the methods developed in this laboratory by Landahl and others with various modifications.